He talked to the civilian Chinese newspaper about the US government hacking Chinese civilian servers.
I trust him, if his motive was really to sell those secrets for money to the Chinese, he would've done it covertly. He wants everyone, not just the Chinese, to have information about what the US Secret Police is doing. Want to bet that there are backups of ALL the files on NSA's illegal activities in the hands of Guardian reporters too? Snowden can disappear at any moment, he'll have trusted someone like Greenwald/a Guardian IT person to take care of his secrets, maybe as an insurance policy as well.
Just like we get pissed if the Chinese hacked Google, the Chinese are pissed that the US hacked into university servers. If it were military targets like the Pentagon, we would think it's fair game...